Hundreds of mothers-to-be a year are being wrongly told they have lost their baby because of mistakes in reading ultrasound scans, doctors fear.

Some of the 400 women given a wrong diagnosis each year will choose to wait to see if they go on to miscarry naturally but others will take the option of terminating the pregnancy.

Last night the researchers said that the possibility of an ‘inadvertent termination’ was the ‘worst possible outcome for any woman’ and called for the guidelines used to determine miscarriages to be changed immediately.

Fears for their unborn child: Mr and Mrs Zubair from Bradford were refused a second scan after being told it was not 'policy'

Professor Tom Bourne, of Imperial College London, said: ‘For most women, sadly there is nothing we can do to prevent a miscarriage.

‘But we do need to make sure we don’t make things worse by intervening unnecessarily in ongoing pregnancies. We hope our work means that the guidelines to determine miscarriage are made as watertight as we would expect for determining death at any other stage of life.’

Around a third of the 500,000 miscarriages a year in the UK are confirmed via ultrasound scans of the foetus and the sac that envelops it in the womb.

The scans from very early pregnancies are particularly hard to read and a second scan will often be carried out to ensure accuracy if a miscarriage is suspected.

For instance, if the foetus is very small and a heartbeat can’t be detected, the woman has a second scan seven to ten days later. The same applies if the gestational sac that should contain the foetus is relatively small and appears to be empty.

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But if the foetus or sac is not unusually small, for example if the sac is over 2cm, just one scan is done before the woman is told she has lost her baby.

She then has the option of waiting to miscarry naturally, taking pills to induce the miscarriage or surgical removal.

But experts say the 2cm cut-off point between having one scan and two is too risky.

A study of more than 1,000 British women estimated that around one in 200 who are deemed to have miscarried because they have an apparently empty gestational sac of over 2cm will actually still be pregnant.

These women would benefit from waiting for a second scan a week or so later that would be expected to reveal the foetus was alive.

A further study revealed this figure could be much higher, as the measurements taken from the scans vary by as much as 20 per cent between medical staff.

Testing time: Mistaken diagnoses in ultrasounds can be caused by misreading complex scans, old equipment and human error

This means that the size of an apparently empty gestational sac could be overestimated, and miss the cut-off point for a second scan, increasing the possibility of the mother undergoing a wrongful termination.

Mistaken diagnoses can be caused by difficulty in reading the complex scans, old equipment and simple human error.

The researchers said the precise numbers are unclear, but writing in the journal Ultrasound In Obstetrics And Gynecology (sic), they estimated that errors could lead to 400 women with healthy pregnancies being wrongly told their baby has died each year.

It is not known how many of these
will go on to have terminations but the figure of 400 compares with 300
cot deaths a year in the UK.

The
research team, from four London hospitals and the Catholic university
of Leuven in Belgium, said: ‘These numbers are significant and relate to
pregnancies that would be highly likely to reach term.’

Professor
Bourne, who led much of the research, called for an immediate interim
change in the guidelines, pending further large-scale research.

He would
like the cut-off point between one scan and two to be raised to 2.5cm
or one inch.

In the meantime, he recommends those
who are anxious to consider requesting the option of returning in a week
to have a second scan.

Dr
Mark Hamilton, a consultant gynaecologist at Aberdeen Maternity
Hospital, said the research reinforced the need for staff to take the
greatest care when determining miscarriage, and urged that medical or
surgical procedures should be postponed until the outcome of the
pregnancy was a ‘certainty’.

The
Royal College of Obstetricians & Gynaecologists said a review of
its guidelines was already under way and it will take the new findings
into account.